Hardening of austenitic chromiumnickel steels by working at subzero temperatures



Oct. 24, 1950 N. A. zlEGLER ETAL 2,527,287

HARDENING 0F AUSTENITIC CHROMIUM-NICKEL STEELS BY WORKING .AT suBzERo TEMPERATURES 2 Sheets-Sheet 1 Filed sept. 2s. 1947 Oct. 24, 1950 Filed sept. 25, 1947 N. A. ZIEGLER ETAL HARDENING 0F AUSTENITIC CHROMIUM-NICKEL STEELS BY WORKING AT SUBZERO TEMPERATURES 2 Sheets-Sheet 2 V PN. 600

AFTER `SHnr ElASr/NG AND S/NG Ar 750% Fm Z4 Hrs.

:g RI Approx/MATE ga rfMPEKArl/AE 0F i: uol/10 /v/RoGEN b @WQ 'I Patented Oct. 24, 1950 l UNITED STATES PATENT 2,527,287 HARnENmG or' AUs'rENrrIc CHROMIUM- 2,527,287 oFFicE NICKEL STEELS vBY WORKING AT SUB- ZERO TEMPERATURES poration of Illinois Application September 23, 1947, Serial No. 775,616 6 Claims. (Cl. 14S-12.3)

This invention relates to a process for hardening austenitc chromium-nickel steels by cold working at sub-zero temperatures.

In order to acquire a better understanding of the background of this invention it should be understood at the outset that austenitc chromium-nickel steels cannot be hardened by any simple thermal method as in the case of ordinary carbon steels or low alloyed steels.

In the hardening of carbon steels or low alloyed steels, the latter phenomenon is a result of the decomposition of austenite. When any of such steels is heated to a temperature higher than its critical transformation on heating, it becomes austenitc. The austenitic state in any steel is associated with" softness, low hardness, high ductility and high plasticity. When cooled to room temperature any carbon steels or low alloyed steels upon passing through their critical temperature on cooling, always transform from austenitc to the ferritic state. For carbon and low alloyed steels this result is universally true, regardless of the cooling rate.

Reference herein is made to a steel, which when processed in conventional ways, is predominantly austenitc vat room temperatures, i. e., the main bulk of the material has a face centered cubic crystal structure.

However, by regulating the cooling rate, any one of the carbon steels or the low alloyed steels can be made either ferrito-pearlitic or ferritomartensitic. The ferrito-pearlitic state is obtained by slow cooling through the transformation temperature on cooling and is associated with relatively low hardness, high ductility and high plasticity. The ferrito-martensitic state is ob.

tained by rapid cooling through the transformation temperature on cooling and is associated with relatively high hardness, low ductility and low plasticity.

By adding to a steel certain alloyed elements such as nickel or manganese, the stability of the austenitc state is increased. Moreover the transformation temperature on cooling at a standard rate (with the increasing percentages of such alloying elements) is suppressed to steadily decreasing values. When the percentage of such alloying element increases beyond a certain figure, the austenitc state may be retained even at room temperatures. In the latter connection for example, a steel containing 25 to 30% nickel is austenitc at room temperatures.

Further by adding to a nickel-bearing steel a third element such as chromium, the stability of austenite is even further increased. In a steel containing 18% chromium, only about 8% nickel is necessary in order to maintain the austenitc state at room temperatures.

The latter is an example of the well-known class of austenitic chromium-nickel steels, prominently known for their corrosion resistance or stainlessnesa 2 It is also well known that the austenitc state in this and related types of steel is metastable." 4'I'he almost fully austenitc state is obtained by relatively rapid cooling from a high temperature such as about 2000 degrees Fahrenheit, but austenite so formed has a tendency to transform into the ferritic state. This transformation can be speeded up by the employment of such other methods, 'such as (1) mechanical working, (2) heating to moderately high temperatures such as 1200-1600 degrees Fahrenheit and (3) submerging to sub-zero temperatures.

For example, the austenitc Hatfield manganese steel (containing about 12% manganese and about 1% carbon) is predominantly austenitc and relatively soft after a high temperature heattreatment, but it becomes martensitic` and hard upon subsequent cold working at room temperatures. In the chromium-nickel austenitc steels related phenomenon have been observed on several occasions. These steels can be hardened to a certain degree by cold working. Austenite can be transformed to some extent to the ferritic state by a moderately high heat` treatment (at about 1000-r 1600 degrees Fahrenheit). Submerging in liquid air promotes in some of these steels the formation of martensite-like structure associated with a certain increase in hardness.

However, the austenite transformation in chromium-nickel steels by any one of the above mentioned methods has never produced any substanappreciated by those skilled in the field of metallurgy. The presentation of such 1Tfcrences has been made as stated in order to provide the background for a better understanding of our discovery, the detailed description of which willhereinafter be given.

We have discovered that the hardness of an austenitc chromium-nickel steel can be increased at least twice its original hardness and frequently considerably higher by cooling such steel to the temperature range between zero degrees Fahrenheit and down to the temperature of liquid air or liquid nitrogen, atA which temperature range the steel is mechanically worked and nally bringing it back to the normal room temperature.

Furthermore we have also discovered that the hardness thus obtained can be increased still further by a subsequent aging heat treatment at a temperature of approximately 750 degrees Fahrenheit.

Corrosion tests conducted on samples treated in accordance with the foregoing procedure show that the corrosion resistance of such samples has not suffered.

For example as hereinafter graphically illustrated a steel of the 18% chromium, 8% nickel type, the chemical composition of which-was as follows: 17.84% chromium, 8.12% nickel and .08% carbon, was shot blasted at the temperature of liquid nitrogen (about minus 300 degrees Fahrenheit). Its surface hardness increased from 180 Vickers to 471 Vickers. Upon subsequent aging at 750 degrees Fahrenheit for 24 hours, the hardness referred to increased lstill further to 545 Vickers.

In connection with the foregoing description attention is directed to the drawings in which:

Fig. 1 is a graphic representation of the increase in hardness in the two different steels due to cold working and subsequent aging.

Fig. 2 is a microstructure of the surface of an 18% chromium, 8% nickel, 2% molybdenum steel, shot blasted at minus 300 degrees Fahrenheit and aged at 750 degrees Fahrenheit. The left hand edge oi the picture is the outer skin, whereas the right hand edge represents the body of the metal. The micro-Vickers hardness impressions indicate the progressive increase in hardness from about 215 Vickers (in the body) to about 465 Vickers (close to the skin) Fig. 3 is a graphic plot of surface hardness values obtained in a steel of 18% chromium, 8% nickel type (a) after shot blasting at various temperatures between room temperature and the temperature of liquid nitrogen, and (b) after shot blasting at various temperatures between room temperature and the temperature of liquid nitrogen, followed by aging heat treatment at I150 degrees Fahrenheit.

In referring to the drawings it should be understood in connection with Fig. 1 that the hardness values given in the test and graphically represented in this figure are averages of a considerable number of individual readings.

As also shown in Fig. 1, the same steel was forged at the temperature of liquid nitrogen (about minus 300 degrees Fahrenheit) and its hardness increased from the original 180 Vickers to 410 Vickers. Upon subsequent aging at 750 degrees Fahrenheit, the hardness referred to increased still further to 470 Vickers.

Another steel of the 18% chromium, 8% nickel, 2% molybdenum type, chemical composition of which was as follows: 18.25% chromium, 8.00% nickel, 2.24% molybdenum, .08% carbon, was shot blasted at the temperature of liquid nitrogen and its surface hardness, as graphically illustrated in Fig. 1, increased from 202 Vickers to 448 Vickers. Upon subsequent aging at 750 degrees Fahrenheit for 24 hours, the hardness referred to was increased still further attaining 553 Vickers.

The same steel was forged at the temperature of liquid nitrogen and, as also shown in Fig. 1, its initial hardness increased from 202 Vickers to 369 Vickers. Upon subsequent aging at 750 degrees Fahrenheit for 24 hours, this hardness increased further to 445 Vickers.

For the sake of comparison the latter steel was similarly cold workedat room temperatures and subsequently aged at 750 degrees Fahrenheit.

Upon shot blasting at room temperature, its surface hardness, as shown in Fig. 1, increased from the original 202 Vickers to 293 Vickers and upon subsequently aging at 750 degrees Fahrenheit for 24 hours the hardness increased to 375 Vickers.

Upon room temperature forging, as shown in Fig. 1, its hardness increased from the original 202 Vickers to 369 Vickers and upon subsequently aging at 750 degrees Fahrenheit for 24 hours a hardness of 346 Vickers was recorded. It will be apparent that hardness values obtained by mechanical working at about minus 300 degrees Fahrenheit are of a different order of magnitude as compared to those obtained by a similar working at room temperature. Likewise subsequent aging also results in higher values of hardness in the samples worked at minus 300 degrees Fahrenheit as compared with those worked at room temperatures.

It is apparent that the mechanical working at room temperature when compared with our ultimate gures causes only a moderate increase in hardness; from the original 175 Vickers to about 350 Vickers. Moreover, subsequent aging at 750 degrees Fahrenheit in the latter case does not produce any additional hardening.

Mechanical working at temperatures below zero degrees Fahrenheit, as for example at minus 50 degrees Fahrenheit causes a more substantial hardening, namely to about 400 Vickers. Subsequent aging at 750 degrees Fahrenheit brings this hardness value to about 550 Vickers.

Mechanical working at minus degrees Fahrenheit and at about minus 300 degrees Fahrenheit (temperature of liquid nitrogen) brings the surface hardness up to about 470 Vickers, whereas subsequent agng at 750 degrees Fahrenheit increases it still further to 510-550 Vickers.

In other words, this drawing demonstrates that, mechanical working at room temperature produces an only moderate increase in hardness and that this hardness is not increased any further or perhaps is increased only slightly by a subsequent heat treatng. Whereas mechanical working in the temperature range from zero degrees Fahrenheit to minus 300 degrees Fahrenheit produces'an appreciable hardening. Moreover the `hardness thus produced may be increased still further by heat treating at about 750 degrees Fahrenheit.y l

It should be understood that the numerical hardness values given in Fig. 3 are averages of a consderable number of individual readings.

The two most popular types of chromiumnickel austenitic steels have been used in the aforedescribed experimental evidence, namely 18% chromium, 8% nickel and 18% chromium, 8% nickel, 2% molybdenum. However, the austenitic state at room temperature in the iron base chromium, nickel alloys can be obtained within a rather wide range of the percentages of these elements. For example, in a steel containing 12% chromium, about 10% nickel is necessary to achieve the austenitic state at room temperature. When the percentage of chromium is raised to about 18%, 4% nickel may sui'lice. When the percentage of chromium is raised beyond'18% up to about 30%, the percentage of nickel again must be raised up to about 20% in order to achieve the austenitic state at room temperature.

The4 percentage of carbon in these types of alloys is usually maintained at or under 0.1 but for pract'cal considerations it may be raised up to about 0.3%.

Molybdenum is frequently added to these types of alloys in amounts up to about 4% for the purpose o." improving the resistance to certain types of corrosion. Since molybdenum tends to promote ferritic state, the percentage of nickel is frequently raised in order to compensate for this eil'ect and to promote the stability of austenite.

Columbium and titanium are sometimes added in amounts of about ten times the percentage of carbon for the purpose of stabilizing carbide. In

such cases the percentage of nickel is likewise raised to compensate for their eiect.

Nitrogen has a stabilizing influence upon the austenitic state similarly to nickel, and its percentage for this purpose is sometimes artificially raised up to about 0.5%. Moreover, austenitic steels of these types always contain certain percentages of other elements commonly found in steels, such as silicon, manganese, sulphur and phosphorus.

It should be understood that only a preferred application of the use of this method has been set forth in the foregoing and the scope of this invention should therefore be measured by the appended claims.

We claim:

1. The process of improving physical properties such as hardness, tensile strength, yield stress and proportional limit of austenitic chromium-nickel steels by cooling said steels within the temperature range of a few degrees below zero Fahrenheit to the temperature of liquid nitrogen, mechanically working in the latter temperature range, and thereafter subjecting the steel to an aging heat-treatment within the temperature range between 500 and about 1000 degrees Fahrenheit, the said steels being characterized by substantial improvement in said physical properties upon subsequent return to room temperature.

2. An austenitic chromium-nickel steel article, the said article being characterized by relatively high surface hardness obtained by cooling the said article to temperatures within the range of a few degrees below zero Fahrenheit to the temperature of liquid nitrogen, mechanically working the said article while exposed to the latter temperature range and subsequently subjecting the said article to an aging heat treatment within the temperature range between 500 degrees and about 1000 degrees Fahrenheit. g

3. The process of improving the physical properties such as hardness, tensile strength, yield stress and proportional limit of austenitic chromium-nickel steels having an `approximate composition of l2 to 30% chromium, 4 to 20% nickel, from a small amount to 4% molybdenum to promote the alpha or ferritic phase, about 0.1% to 0.3% carbon and the balance substantially iron containing the general ingredients normally present in steels, the steps including the cooling of said steels within the temperature range of a few degrees below zero Fahrenheit to the temperature of liquid nitrogen, mechanically working in the latter temperature range, and thereafter subjecting the steel to an aging heat-treatment within the temperature range between 500 and about 1000 degrees Fahrenhit, the said steels being characterized by substantial improvement in said physical properties uponsubsequent return to room temperature.

fi. The process of improving the physical properties such as hardness, tensile strength, yield stress and proportional limit of austeriitic `chromium-nickel steels, having an approximate composition of l2 to 30% chromium, 4 to 20% nickel, about 0.1% to 0.3% carbon, titanium from a small amount to about ten times the percentage of carbon to stabilize carbides, and the balance substantially iron containing the general ingre-v dients normally present in steels, the steps in binding the cooling of said steels with the tenaperature range of a few degrees below zero Fahrenheit to the temperature of liquid nitrogen, mechanically working in the latter temperature range, and thereafter subjecting the steel to an aging heat-treatment within the temperature range between 500 and about 1000 degrees Fahrenheit, the said steels being characterized by substantial improvement in said physical properties upon subsequent return-to room temperature. n

5. The process of improving the physical properties such as hardness, tensile strength, yield stress and proportional linut of austenitic chromium-nickel steels having an approximate composition of l2 to 30% chromium, 4 to 20% nickel, about 0.1% to 0.3% carbon, columbium from a small amount to about ten times the percentage 0f carbon for stabilizing carbides, the balance substantially iron containing the general ingredients normally present in steels, the steps including the cooling of said steels within the temperature range of a few degrees below zero Fahrenhit to the temperature of liquid nitrogen, mechanically working in the latter temperature range, and thereafter subjecting the steel to an aging heat-treatment within the temperature range between 500 vand about 1000 degrees Fahrenheit, the said steels being characterized by substantial improvement in said physical properties upon subsequent return to room temperature.

6. The process of improving the physical properties such as hardness, tensile strength, yield stress and proportional limit of austenitic chromium-nickel steels having a-n approximate composition oi' l2 to 30% chromium, 4 to 20% nickel, from a small amount to 0.5% nitrogen to promote the stability of austenite, about 0.1% to 0.3% carbon and" the balance. substantially irony containing the general ingredients normally present in steels, the steps including the cooling of said steels within the temperature range of a few degrees below zero Fahrenheit to the temperature of liquid nitrogen, mechanically working in the latter temperature range, and thereafter subjecting the steel to an aging heat-treatment within the temperature range between 500 and about 1000 degrees Fahrenheit, the said steels being characterized by substantial improvement in said physical properties upon subsequent return to room temperature.

NICHOLAS A. ZIEGLER.

WILBUR L. MEINHART.

JAMES R. GOLDSMITH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,427,100 Gilbert Aug. 29, 1926 1,931,013 Ennor Oct. 24, 1933 2,020,728 Lowry et al. Feb. 4, 1930 2,270,762 Morrill Jan. 20, 1942 OTHER REFERENCES "Transactions of the Institute of Chemical Engineers, published by the Institute, London, i933, vol. il., pp. 89-106 and 122.

Alloys oi Iron and Chromium, voi. 2 by Kinzel and Franks, McGraw-Hill Book Co., N. Y., i900, page .inchiv iur das Eisenhuttenwesen, vol. 6 pp. titi-Siti. 

1. THE PROCESS OF IMPROVING PHYSICAL PROPERTIES SUCH AS HARDNESS, TENSILE STRENGTH YIELD STRESS AND PROPORTIONAL LIMIT OF AUSTENITIC CHROMIUM-NICKEL STEELS BY COOLING SAID STEELS WITHIN THE TEMPERATURE RANGE OF A FEW DEGREES BELOW ZERO FAHRENHEIT TO THE TEMPERATURE OF LIQUID NITROGEN, MECHANICALLY WORKING IN THE LATTER TEMPERATURE RANGE, AND THEREAFTER SUBJECTING THE STEEL TO AN AGING HEAT-TREATMENT WITHIN THE TEMPERATURE RANGE BETWEEN 500 AND ABOUT 1000 DEGREES FAHRENHEIT, THE SAID STEELS BEING CHARACTERIZED BY SUBSTANTIAL 